Photoelectric cell circuit



Aug. 3, 1948. A WOLFET`| v Re. 23,023

PHOTOELECTRIC CELL CIRCUIT Original Filed April 22, 1944 $9 8f 7j l f and the like.

thus constitutes one calibration point, and others may be obtained in the same manner by the use of additional screens of standardized transmission values.

The calibration of potentiometer 4, as clescribed above, represents a. correlation of the balancing resistance value (R4 bal.) and the ratio of the illumination reaching photocell 2 to that reaching cell I. values, as discussed below, this correlation becomes, within the accuracy required for most' measurements,

Illumination of cell 2 C, R3+R4 bal. Illumination of een 1 f R3+R4 total where C is a constant related to the characteristics of the photocells. Since this relationship involves th'e light intensities only in the form of a With proper choice of circuit y ratio, the calibration of the potentiometer IIv is Y independent of the absolute values of the iight intensities. This characteristic of our circuit is ofreat value in applications .to instruments such as colorimeters, turbidometers, densitometers, The calibration of instruments of this rvtype which incorporate our dual photocell circuit may be maintained substantially constant, yirrespective of changing characteristics of the light source due to age deterioration, line voltage uctuation, and the like.

lThe. maintenance of calibration, irrespective of change in absolute light values over a wide range, is illustrated by the following specic example:

Example f'rne circuit of Fig. 1 of the drawing is emp loyed with the Ifollowing resistance values:

. Ohms Rltotal s 22 R5 1 25 The photocells comprise blocking layer cells having the following characteristics:

Open Circuit Po- Short Circuit Curi f l Illumination,

tential, mllivolts rent, microamperes loot-candles Cell 1 Celli! Cell 1 Cell 2 Resistances in Galvanometer Illumination Illumination Circuit at of Cell l, of Cell 2, alance, ohms foot-candles foot-candles Rs Rs R4 Our circuit is adapted for use with any photoelectric elements of the type which transform light energy to electrical energy, and al1 such elements are referred to herein as photocells. The range of absolute `light intensities within 4 which relative intensity measurements can be made with our circuit will, of course, depend on the characteristics of the particular photocells employed. In 4any case, however, it is desirable to Operate in the intensity range over which the power output of the cell is substantially linear with respect to illumination. We prefer to employ current generating photocells of the barrier vlayer or blocking layer type, and with such cells the absolute light intensities should be below 50 foot-candles at the cell surface, and preferably 1% to about 10% of the cell internal resistance.V If the total resistance of the calibrated poten-f tiometer 4II is xed, the range of measurable illumination ratios will be from to 100 R3l t RS4-R4 total Per en of the zero-point ratio. Ii the photocells have` substantially identical characteristics, the lowest measurable ratio will also be equal to R5 per cent of the zero-point ratio. It is generally most COD- venient to pre-set R5 to the desired sensitivity, and then adjust R3 for zero-point balance.

It will be evident from the above description that our photocell circuit has advantages which' make it particularly desirable for colorimeter applications in continuous analysers, recorders, controllers, and the like.' One such application is illustrated diagrammatically in Fig. 2 of the ac' companying drawing. In thisl modification of our invention, oui` dual photocell circuit isincorporated in an automatic continuous oxygen analyzer which is adapted to control associated instruments .such as continuous recorders, flow control devices, and the like.

Referring to Fig. 2, light from lamp I0 passes through lenses II and I2 and through absorption cells I3and I4 to photocells I and 2. These' cells are connected in a bridge circuit 4with adjustable resistance 3, potentiometerl, a second adjustable resistance 5, and a galvanometer 6; in the same manner as in Fig. l which has previously been described. In this case, the galvanometer 6 is equipped with contacts I5 and I6 with which the galvanometer needle I'I may complete circuits from lbattery I8 to relay I! when'` the photocell circuit is unbalanced. The rel-ay I9, in turn, serves to connect a power source to a reversible motor 20. The motor 20through suitable driving means such as gears 2|, 22, and threaded shaft 23, propels the traveler 24 which carries the sliding contact 8 of potentiometer 4i The circuit described above is lbalancedl for zero-point with no a'bsorptive media in the absorption cells I3 and I4. On the introduction of a medium of greater absorption in cell I4 than in cell I3, the circuit will automatically reach a new balance, and Vthe proportion of R4 in the bridge circuit will constitute a measure of the relative absorption in cell I3 to thatincell I4.

When this apparatus is employ-ed for continu-- ous oxygen analysis of a gas mixture, as illustrated in Fig. 2, the mixture to be analysed is charged at a constant rate, .and is first passed through absorption cell I3. As a resul-t, the light transmitted through the charge mixture will serve as a reference standard for the photocell circuit. Nitric oxide, in exc-ess oi the amount required to react with all of the oxygen in the chargev gasYA is introduced at a constant rate into the gas stream flowing from absorption cell I3. The gases then flow through a suitable mixing chamber, such as the capillary tube 25 shown in the drawing, The oxygen and nitric oxide in the` gas mixture react to form nitrogen dioxide', and the resulting mixture then flows through the absorption cell M.

The light absorption in cell H by the colored' gas, nitrogen dioxide, will be proportional to the amount of oxygen in the charge gas, and the potentiometer 4 may thus be calibrated directly in terms of oxygen content of the gas mixture. This calibration can, of course, be effected by charging gas mixtures of known analyses. If the range of oxygen content of `the charge gas is known, the sensitivity of the instrument may be pre-set by adjusting the resistance 5 (and 1re-balancing for zero-point) so that the whole range of the potentiometer 4 is utilized for the range of oxygen -content of the gas mixture to be analysed.

It is evident that the instrument described above will automatically and continuously analyze a gas mixture for oxygen cont-ent, and indicate the analysis by the position of the traveler 24 and the sliding contact 8 of the potentiometer 4. .An auxiliary lever 26, carried by the traveler 24, may serve as the actuating member of a suitable linkage for .controlling the pen of a continuous recorder, or for actuating relays for the remote control of oxygen or air valves. Alternatively, the arm 2E may carry electrical contacts of relay Iactuating circuits f or remote flow control devices. A predetermined time cycle of varying oxygen content of the charge gas may thus be controlled by means of cam-driven movable contacts, cooperating with contacts carried by the arm 2S, for actua-ting the relays.

Various other modications of our invention will, of course, be evident to those skilled in the art, and it is to be understood that the scope of our invention is in no way limited to the particur lar modiiications illustrated in the drawings and discussed above. Any equivalents of either the apparatus elements or the electrical circuits described herein may be employed Without departing from the scope of our invention. Only such limitations should be imposed on 'the scope of this invention 'as are indicated in the appended claims.

We claim:

1. In a photocell circuit adapted for the measurement of the relative intensities of a plurality of light beams, the combination of a reference photocell, an adjustable shunt resistance across said reference cell, a second photocell, a second adjustable shunt resistance across said. second cell, the total shunt resistance across each of said photocells being substantially less than the minimum operating internal resistance of the cell, a bridge circuit connecting the shunt resistance of said second cell and a continuously variable portion of the shunt resistance of said reference cell ranging from the tota-l shunt resistance of said reference cell to a fraction thereof so that the potentials across said resistances will be in op- 6 position, and an indicating device in said" bridge circuit adapted to indicate balance or uri-balancev of said potentials.

2. In a photocell circuit adapted'ior the measurement of the relative intensities of a plurality of light beams, the combination of a reference photocell, ashunt resistance across said reference cell, said shunt resistance comprising an adjustable resistance in series with the total resistance of 4a variable potentiometer, a second photocell, an adjustable shunt resistance across said second cell, the total shunt resistance across each of said photocells being less than 10% of the minimum operating internal resistance of the` cell, a bridge circuit connecting the shunt resistance of said second cell, the adjustable resistance across said reference cell and the variable resistance of said potentiometer, and an indicating device in said bridge circuit adapted to indicate balance or unb-alance of said potentials.

3. In a photoelectric measuring device adapted for the measurement of the relative absorption of light from two beams emitted from the same source, the combination of a light source, a reference photocell, a reference absorption cell interposed between said light source and said reference photocell, a second photocell, a second absorption cell interposed between said light source and said second photocell, a shunt resistance across said reference photocell comprising an adjustable resistance in series with the total resistance of a variable potentiometer, said potentiometer having a sliding contact, means for driving said sliding contact, an adjustable shunt resistance across said second photocell, a bridge circuit connecting the shunt resistance of said second cell, the adjustable resistance and the variable resistance of said potentiometer so that the potential across said first named shunt resistance will be opposed to the potential across said adjustable resistance and said variable resistance, means responsive to the current in said bridge circuit for actuating the driving means for the sliding contact of said potentiometer to effect a. balance of said potentials, and means connected to said sliding contact for indicating the position of said sliding contact and thus the relative light absorption in said absorption cells.

4. In a photoelectric measuring device adapted for the measurement of the relative absorption of light from two beams emitted from the same source, the combination of a light source, a reference photocell, a reference absorption cell interposed between said light source and said reference photocell, a second photocell, a second absorption cell interposed between said light source and said second photocell, a shunt resistance across said reference photocell comprising an adjustable resistance in series With the total resistance of a variable potentiometer having a sliding contact and means for driving said sliding contact, an adjustable shunt resistance across said second photocell, the total shunt resistance across each of said photocells being substantially less than the minimum operating internal resistance of the cell, a bridge circuit connecting the shunt resistance of said second cell, the adinstable resistance and the variable resistance of said potentiometer so that the potential across said first named shuntresistance will be opposed to the potential across said adjustable resistance and said variable resistance, means responsive to the current in said bridge circuit for actuating the driving means for the sliding contact of said potentiometer to effect a balance of said potentials, and means connected to said sliding contact for indicating the position of Asaid sliding contact and thus the relative light absorption in said absorption cells.

5. In a photoelectric measuring device adapted for the measurement ofzthe relative absorption of light from two beams emitted from the same source, the combination of a light source, a reference photocell, a reference absorption cell interposed between said light source and said reference photocell, a secon-d photocell, a second absorption cell interposed between said light. source and said second photocell, a shunt resistance across said reference photocell compris-v ing an adjustable resistance in series with the total resistance of 'a variable potentiometer having a sliding contact, and means for driving said sliding contact, an adjustable shunt resistancey Iacross said second photocell, the total shunt-resistance across each of said photocells being less than 10% of the minimum operating internal resistance of the cell, a bridge circuit connecting the shunt resistance of said second cell, the adjustable resistance and the variable resistance of said potentiometer so that the potential across said first named shunt resistance will be opposed to the potential across said` adjustable resistance and said variable resistance, means responsive to the current in said bridge circuit for actuating the driving means for the sliding contact of said potentiometer to effect a balance of said potentials, and means connected -to said sliding contact for indicating the position of said sliding contact and thus the relative light absorption in said ab-l sorption cells.

ALEXANDER WOLF.

GERHARD HERZOG. 

